Vacancy formation, stability, and electronic properties of nickel on equimolar ceria–zirconia mixed oxide (111) catalyst

Abstract

Nickel (Ni) single atoms and small clusters dispersed on Ce_0.5Zr_0.5O₂ (CZO) are promising for the dry reforming of methane (DRM). However, understanding of defects and electronic interactions between Ni motifs and (non) stoichiometric CZO surfaces is limited. Here, we use Density Functional Theory (DFT) and ab initio thermodynamics to investigate vacancy formation and electronic properties of Ni₁ and Ni₄ on CZO(111). Surface and subsurface oxygen vacancies near Zr⁴⁺ ions dominate due to distortion-induced stabilization across DRM-relevant chemical potentials; vacancies prefer to be subsurface. Interestingly, Ni single atoms coordinate with two O_Zr surface atoms on CZO(111) (Ni_Ce2Zr), unlike three-fold coordinated Ni atoms on hollow sites of CeO₂(111). Ni₁ does not directly bind to oxygen vacancies due to its oxophilicity and steric hindrance caused by multiple surface Ce³⁺ ions. Clusters, on the other hand, can bind favorably at a surface oxygen vacancy. Ni adatoms are more stable than Ni₄ at trimer defects comprising one Ce and two oxygen vacancies (Ce_v(O_v)₂), at the pristine surface, and at the Ni_Ce2Zr site with a next-nearest neighbor oxygen vacancy due to coordination with more oxygen atoms. The extent of electron transfer from the metal to the surface and, thus, the degree of cationic character of a nickel adatom varies with its location and defect type and correlate positively with its resistance to sintering. We discuss the expected heterogeneity of actual catalysts.